Category Archives: populations

The clue of the missing jet

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The cosmic neighbor that kept us in the dark

Some X-ray binaries scream for attention, others just quietly hum along. 4U 1556–60 is one belonging to the latter category, known for over 50 years and thought to be a run-of-the-mill X-ray binary system living near the center of our Galaxy, like so many. But as it turns out, we were looking at it all wrong.

A surprising shortcut was delivered by the Gaia satellite, which measures the distances to billions of stars with incredible precision. Distilling the optical counterpart of 4U 1556–60 from the many stars Gaia has detected, we found that Instead of being roughly 25,000 light-years from Earth, this X-ray binary must actually be no less than ten times closer! At a distance of only 700 pc, it is one of the closest X-ray binaries ever discovered and this proximity drastically alters everything we thought we knew about it. To put it in perspective, it’s like thinking a faint light in the distance that you’ve seen forever is a massive lighthouse miles away, only to realize it’s a tiny candle right across the street.

Because it is so much closer, we now know it is 100 times fainter than previously believed. Its persistent low luminosity is quite unusual for an X-ray binary: Usually, these systems either flare up brightly or remain dark in a state of quiescence, but 4U 1556–60 just quietly simmers at a very low level. To figure out if the system’s heart is a black hole or a neutron star, we used the Australia Telescope Compact Array (ATCA) to look for a radio jet. The result was absolutely nothing: one of the lowest radio limits ever recorded for an X-ray binary.

While black holes are notoriously noisy in radio waves, neutron stars produce much weaker jets, even when their X-ray emission shines similarly bright. The deep radio silence is the strongest evidence yet that 4U 1556-60 hosts a neutron star. Despite that no thermonuclear bursts, smoking-gun evidence for the presence of a neutron star, have ever been detected from the source this is likely because it is eating its companion so slowly that it takes months or even years to build up enough fuel for a single pop. The very low X-ray luminosity and low fueling rate likely implies that the neutron star in 4U 1556-60 is orbiting very closely to its companion that is just a tiny, stripped-down remnant of a star. While faint in X-rays, such ultra-compact X-ray binaries might actually be screaming in gravitational wave emission. Due to its close proximity, 4U 1556-60 is therefore a prime target for future gravitational wave detectors like LISA.

Pattie, Maccarone, Russell, Bachetti, Degenaar, Kupfer 2026, A&A 707, 193: 4U 1556-60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

Paper link: NASA ADS

Radio and X-ray brightness of a large number of black hole and neutron star X-ray binaries, with our close neighbor 4U 1556-60 indicated as the red star, showing its comparatively faint X-ray emission and missing radio jet.

New kid on the block

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The center of our Milky Way is like the cosmic Times Square; a crowded, chaotic neighborhood where stars are closely packed and a supermassive black hole, Sgr A*, is the center of attention. For over 20 years, the Swift satellite has been our eyes on the ground, taking daily X-ray snapshots to catch the sudden activation of otherwise quiet inhabitants in this busy region. In February 2024, suddenly a shy new face appeared in the crowd: a faint X-ray source popped up and now carries the name Swift J174610−290018.

Soon after it stuck its head above the crowd, it became clear that Swift J174610−290018 was a member of the X-ray binary class of objects. These are binary star systems wherein a neutron star or a black hole devours its companion star through a process that we call accretion. However, while most X-ray binaries draw attention by shining very brightly in X-rays, this new kid on the block appeared much more shy, radiating only faint X-ray emission. This sub-set of X-ray binaries are called Very Faint X-ray Transients (VFXTs) and they appear to take only light bites instead of full meals, which makes them much harder to spot and to study in detail. Swift J174610−290018 showed up for about 50 days in 2024, only to vanish and reappear for a brief, five-day rendez-vous in April 2025,

Figuring out the exact nature of Swift J174610−290018, for instance if it contains a black hole or a neutron star, required some serious detective work in the X-ray data archives of NASA and ESA. Digging through data from the XMM-Newton satellite, we found a single, bright X-ray flare coming from the position of Swift J174610−290018 that had occurred in 2004 and hence had been hiding in the archives for two decades. This X-ray flare had all the hallmarks of a thermonuclear X-ray burst: a sudden thermonuclear explosion on the surface of an accreting neutron star. Such an event is the ultimate ID card showing that Swift J174610−290018 most host a neutron star, because black holes don’t have a solid surface to catch falling gas and hence cannot trigger these kinds of explosions.

Very-faint X-ray binaries are among the Galactic center’s most elusive inhabitants but may well represent the largest population of X-ray binaries in the Milky Way. For this reason finding, identifying and characterizing them, as we did for Swift J174610−290018, remains an important endeavor to understand the make-up of our Galaxy and to understand how pairs of binary stars live and end their lives. The discovery of this new X-ray source is another win for long-term monitoring programs which have proven very effective in catching these faint transients and allowing to trace their history across decades of data. Swift J174610−290018 also proves that even in a region we’ve watched daily for 20 years, there are still new neighbors waiting to be introduced.

Stel, Ponti, Degenaar, Sidoli, Mereghetti, Mori, Bao, Illiano, Mondal, Reynolds, Jin, Lian, Mandel, Scaringi, Zhang, Sanger-Johnson, Wijnands, Miller, Kennea, Zhu 2026, Astronomy & Astrophysics 705, 135: The very faint X-ray transient Swift J174610-290018 at the Galactic center

Paper link: SciX

Neighborhood map showing newly the discovered very-faint X-ray transient Swift J174610–290018 nested in the crowded hearth of our Milky Way’s center, spatially close to our supermassive black hole Sgr A*. Shown is the accumulated X-ray image from the EPIC-pn camera onboard the XMM-Newton satellite taken in the spring of 2024. Swift J174610–290018 is located 6.7 arcmin east of Sgr A*. The neutron star X-ray binary AX J1745–2901 is often active and can also be clearly seen in this image, being the brightest X-ray source in the field of view. The inset displays the position uncertainties of the newly identified 2024 transient (green circle) and the positions from previously cataloged X-ray sources reported.